This invention relates to a method and apparatus for separating argon and oxygen from oxygen-enriched air.
The most important method commercially for separating air is by rectification. In typical air rectification processes, there are performed the steps of compressing a stream of air, purifying the resulting stream of compressed air by removing water vapour and carbon dioxide from it, and precooling the stream of compressed air by heat exchange with returning product streams to a temperature suitable for its rectification. The rectification is performed in a so-called "double rectification column" comprising a higher pressure and a lower pressure column, i.e. one of two columns operates at a higher pressure than the other. Most of the incoming air is introduced into the higher pressure column and is separated into oxygen-enriched liquid air and a nitrogen vapour. The nitrogen vapour is condensed. Part of the condensate is used as liquid reflux in the higher pressure column. Oxygen-enriched liquid is withdrawn from the bottom of the higher pressure column and is used to form a feed stream to the lower pressure column. Typically the oxygen-enriched liquid stream is sub-cooled and introduced into an intermediate region of the lower pressure column through a throttling or pressure reduction valve. The oxygen-enriched liquid air is separated into substantially pure oxygen and nitrogen in the lower pressure column. Gaseous oxygen and nitrogen products are taken from the lower pressure column and typically form the returning streams against which the incoming air stream is heat exchanged. Liquid reflux for the lower pressure column is provided by taking the remainder of the condensate from the higher pressure column, sub-cooling it, and passing it into the top of the lower pressure column through a throttling (i.e. pressure reducing) valve. An upward flow of vapour through the lower pressure column from its bottom is created by reboiling liquid oxygen. The reboiling is carried out by heat exchanging the liquid oxygen at the bottom of the lower pressure column with nitrogen from the higher pressure column. As a result, the nitrogen vapour is condensed.
A local maximum concentration of argon is created at an intermediate level of the lower pressure column beneath that at which the oxygen-enriched liquid air is introduced. If it is desired to produce an argon product, a stream of argon-enriched oxygen vapour is taken from a vicinity of the lower pressure column where the argon concentration is typically in the range of 5 to 15% by volume of argon and is introduced into a bottom region of a side column in which an argon product is separated therefrom. Typically, no steps are taken to adjust the pressure of the argon-enriched oxygen vapour stream as it flows from the lower pressure column to the argon column. Reflux for the argon column is provided by a condenser at the head of the column. The condenser is cooled by at least part of the oxygen-enriched liquid air upstream of the introduction of such liquid air into the lower pressure column.
It is well known to use sieve trays in the argon column in order to effect contact between liquid and vapour therein. Since argon and oxygen have similar volatilities, a considerable number of trays are typically used in the argon column. The resulting pressure drop in the argon column has the result that a desirable small temperature difference can be maintained between argon being condensed and the oxygen-enriched liquid used to cool the head condenser.
Since the middle of the 1980's considerable interest has been focused upon using packing instead of trays in order to effect liquid-vapour contact in the columns of an air separation plant. EP-A-0 377 117 confirms that by using a sufficient height of packing in the argon column an essentially oxygen-free argon product can be taken from it. (If distillation trays are used in the argon column, the pressure drop is sufficient for,the condensing temperature of oxygen-free argon to become so low that the head condenser would become inoperable when it is required to introduce the oxygen-enriched fluid from it into the lower pressure column.) However, as a result the temperature difference between the oxygen-enriched liquid and the argon streams in the head condenser becomes undesirably high. EP-B-341 512 discloses controlling the pressure difference in the head condenser by employing a valve to reduce the pressure of the argon-enriched oxygen stream flowing from the lower pressure column to the argon column. EP-A-594 214 discloses a process in which the argon-enriched oxygen is used to reboil the argon column, being condensed thereby. The condensed argon-enriched oxygen stream is introduced into the argon column at an intermediate mass transfer region thereof but liquid is still returned from the bottom of the argon column to the same region of the lower pressure column from which the argon-enriched oxygen is withdrawn.
In all the processes described above, the performance of that part of the separation in which the argon concentration of the oxygen is reduced from 5% by volume to that specified for the oxygen product is performed exclusively in the lower pressure rectification column. It is an aim of the present invention to provide a method and apparatus that enables some of this separation to be performed in the argon column itself and an oxygen product to be withdrawn therefrom. Certain advantages are thereby made possible as will be described below.